fix: improve oras manifest fetch --platform error message

Signed-off-by: Zoey Li <zoeyli@microsoft.com>

Author: lizMSFT

copy_test.go

@@ -978,7 +978,7 @@ func TestCopy_WithTargetPlatformOptions(t *testing.T) {
 	}
 
 	_, err = oras.Copy(ctx, src, ref, dst, "", opts)
-	expected = fmt.Sprintf("mismatch MediaType %s: expect %s", docker.MediaTypeConfig, ocispec.MediaTypeImageConfig)
+	expected = fmt.Sprintf("fail to recognize MediaType %s: expect %s", docker.MediaTypeConfig, ocispec.MediaTypeImageConfig)
 	if err.Error() != expected {
 		t.Fatalf("Copy() error = %v, wantErr %v", err, expected)
 	}

internal/platform/platform.go

@@ -118,7 +118,7 @@ func SelectManifest(ctx context.Context, src content.ReadOnlyStorage, root ocisp
 // fields in config blob.
 func getPlatformFromConfig(ctx context.Context, src content.ReadOnlyStorage, desc ocispec.Descriptor, targetConfigMediaType string) (*ocispec.Platform, error) {
 	if desc.MediaType != targetConfigMediaType {
-		return nil, fmt.Errorf("mismatch MediaType %s: expect %s", desc.MediaType, targetConfigMediaType)
+		return nil, fmt.Errorf("fail to recognize MediaType %s: expect %s", desc.MediaType, targetConfigMediaType)
 	}
 
 	rc, err := src.Fetch(ctx, desc)